When you initially create a PO, items are ordered as they are in Products & in Stock Central. I say "initially" because, if you go back and add an item later, it stays at the bottom regardless of its position in Stock Central.
The miss here is that variable products are sorted using the product name so, within that name, variations are all lumped together and the variations are what I'm trying to sort on the PO. I admit that example gets into a more specific use-case.
Aside from that specific situation, I do feel like being able to sort items on a PO is a fairly basic feature that would be useful/expected with inventory management software.